The Buddhist Society of London is one of the oldest Buddhist
organisations outside Asia

So were there other groups of western born Buddhists practicing together even earlier than this or would this truly have been the first such grouping?

For the sake of this question I am taking sangha to mean any sort of Buddhist community. I appreciate it often refers specifically to a monastic community however I want to use a broader definition here.

3 Answers
3

One of the problems in answering is in deciding what would qualify as a Buddhist community, specifically because the word "Buddhist":

Is a relatively modern and Western invention

May not mean today what it meant 100 or so years ago (i.e. around the time of the
suspected first Western sangha), and

Still doesn't have a universally
agreed definition (even taking refuge in the three jewels doesn't
cut it in this context)

With that in mind, you could (given your apparent scholarly bent) bring the Theosophical Society -- 1875 -- into your considerations, as at least being a sangha "precursor", especially given Evan-Wentz's involvement. Clearly it was, by no stretch of the definition, what we'd call "Buddhist" today, but would even the first Buddha and his companions have thought of themselves as "Buddhist" by our contemporary definition? More likely they'd have considered themselves Hindu (although even Hindu is a term which was adopted by late 18th century Europeans to refer to the followers of Indian religions).

But only to study Buddhist philosophy was for some Germans not enough. They wanted to organize themselves in Buddhist communities in order to practice Buddhism and to promote Buddhist teaching. In 1903 Karl Seidenstucker (1876-1936) founded the first German Buddhist organisation in Leipzig, called "The Buddhist Mission in Germany" and published in magazine, ‘The Buddhist’.

The paragraph before that mentions various Germans who at least studied it in the 19th century.